PHP检查表单提交的来源

一下代码来自 《PHP5完全攻略》

<?php
$action = $_SERVER['PHP_SELF'];
if($_SERVER['REQUEST_METHOD'] == 'POST'){
    $ref = $_SERVER['HTTP_REFERER'];
    $srv = "http://{$_SERVER['SERVER_NAME']}$action";
    echo "当前来源为:<b>$ref</b><br>服务器地址为:$srv<br />";
    if(strcmp($srv, $ref) == 0){
        echo '匹配';
    }else{
        echo '不允许外站提交';
    }
}else{
    echo '提交表单';
}


?>

<form action="<?php echo $action;?>" method="post">
<input type="submit" value="提交" />
</form>